1. Field of the Invention
The present invention relates to a metal foil-clad laminate using a low dielectric polyimide having a good heat resistance and a good adhesion property. The metal foil-clad laminate is applicable to printed wiring boards, surface heating elements, electromagnetic shielding materials, flat cables, etc.
2. Description of the Prior Art
Some metal foil-clad laminates are produced by bonding an insulating substrate and a metal foil through an adhesive or an adhesive film. For example, there has been proposed a meta foil-clad laminate having a three-layered structure which is formed by bonding an aromatic polyimide film as an insulating substrate to a metal foil through an adhesive film (e.g., Japanese Patent Application Laid-Open No. 55-91895).
Hitherto, epoxy-based or acryl-based adhesives have been mainly used as the adhesive or adhesive film. However, since these resins are poor in heat resistance, products obtained using these adhesives show an insufficient heat resistance, resulting in limitations to subsequent processing conditions and use conditions.
Under these circumstances, there is a demand for adhesives and adhesive films having an excellent heat resistance. For example, there is disclosed a method of forming a thermopress-bonding adhesive layer on an insulating substrate by applying a polyimide or polyamic acid dispersion on the substrate as the adhesive and then removing the solvent, followed, if required, by imidation (e.g., Japanese Patent Application Laid-Open Nos. 5-32950 and 5-59344). Also, there is disclosed a method of forming a thermopress-bonding adhesive film by applying a resin dispersion on a glass plate, etc. and then removing the solvent, followed, if required, by imidation.
To the formed adhesive layer or adhesive film, an adherend such as metal foil is thermopress-bonded. (e.g., Japanese Patent Application Laid-Open Nos. 5-32950 and 5-59344 and Japanese Patent No. 3213079).
Further, there is known a method of forming an insulating protective film layer on a circuit surface of a printed wiring board by a liquid cover coat agent (a cover lay ink) or a cover lay film. In this method, a cover coat agent is applied to a circuit surface of a printed wiring board by a method such as screen printing and then subjected to a treatment such as curing to from a cover coat layer. However, since the conventional cover coat agents extensively used are mainly made of epoxy-based resins that are poor in heat resistance and flexibility, the printed wiring board having a cover coat also tends to be insufficient in heat resistance and flexibility, resulting in limitations to subsequent processing conditions and use conditions. Therefore, it has been demanded to provide cover coat agents having excellent heat resistance and flexibility. For example, there has been proposed a method of forming a cover coat layer on a circuit surface of a flexible printed wiring board by applying a dispersion of polyimide or polyamic acid in a dispersing medium such as solvent on the circuit surface and then removing the solvent, followed, if required, by imidation (e.g., Japanese Patent No. 2820497 and Japanese Patent Application Laid-Open No. 8-109259).
Polyimide films having an excellent heat resistance are frequently used as a cover lay film. However, many of the polyimide films themselves are less adhesive to require the use of additional epoxy-based or acryl-based adhesives that are poor in heat resistance, causing the same problems as described above. Thus, there is a demand for adhesive films that are excellent in both flexibility and heat resistance. For example, there has been proposed a method of forming a thermopress-bonding, adhesive cover lay film by applying a dispersion of polyimide or polyamic acid on a support for film formation such as a glass plate and a metal plate, and then removing the solvent, followed, if required, by imidation (e.g., Japanese Patent No. 3213079).
In the recent application fields such as information processing and telecommunications, transmission frequency or operating frequency of CPU becomes higher in order to transmit or process a large content of information data. Therefore, it is required to shorten the time delay of signal transmission rate by reducing the dielectric constant of a whole insulating layer including an adhesive layer and a cover coat layer in addition to making the insulating layer thinner. However, since the above conventional adhesive layers, adhesive films, cover coat layers and cover lay films are made of aromatic polyimides, the dielectric constant at 10 GHz unfavorably becomes as high as about 3.5, although varies depending upon the content of aromatic ring.
It has been known that the dielectric constant of a polymeric material can be generally reduced by using an aliphatic monomer. The inventors have produced various polyimides from non-aromatic tetracarboxylic dianhydrides. Examples of the non-aromatic tetracarboxylic dianhydride include aliphatic (chain) tetracarboxylic dianhydrides such as 1,2,3,4-butanetetracarboxylic dianhydride, and alicyclic tetracarboxylic anhydrides such as 1,2,3,4-cyclobutanetetracarboxylic dianhydride, 1,2,4,5-cyclopentanetetracarboxylic dianhydride and bicyclo[2.2.2]octa-7-ene-2,3,5,6-tetracarboxylic dianhydride. However, polyimides obtained from the aliphatic (chain) tetracarboxylic dianhydride have an extremely low heat resistance and are inapplicable to the processing such as welding, reducing its practical utility. On the other hand, polyimides obtained from the alicyclic tetracarboxylic anhydride have an improved heat resistance as compared to those obtained from the chain tetracarboxylic dianhydride. However, since polyimides obtained from 1,2,3,4-cyclobutanetetracarboxylic dianhydride have a low solubility to solvents, the application of a solution of such polyimides onto a metal foil, an insulating substrate or a support for film formation fails to form a polyimide film having a thickness sufficient for the use as the adhesive layer, cover coat layer of cover lay film. Although 1,2,4,5-cyclopentanetetracarboxylic dianhydride and bicyclo[2.2.2]octa-7-ene-2,3,5,6-tetracarboxylic dianhydride provide polyimides having a high solubility to solvents, the film obtained by applying the polyimide solution onto a metal foil, an insulating substrate or a support for film formation fails to have a flexibility, reducing the practical utility as the adhesive layer, cover coat layer and cover lay film.